There is some data out there that suggests that sex does affect a woman's cycle. A woman who has sex once a week or more is more likely to ovulate on a regular basis. If you have just recently started having sex, the emotional stress that goes along with that could cause you to ovulate later, not at all, and both of those things would cause a late period. An earlier than usual period from time to time is also normal in the grand scheme of things.

Of course, the most obvious reason that sex could cause a late period is pregnancy. If you are sexually active and have missed a period, it would probably be a good idea to take a pregnancy test. The home tests are very accurate when performed correctly. No birth control is 100%. I have a five-year-old condom baby playing in his room right now.